1. Field of the Invention
The present invention pertains to the field of telecommunications, and particularly to frequency hopping techniques employed in cellular telecommunications systems.
2. Related Art and Other Considerations
Frequency hopping is often employed in cellular, radio telecommunications systems, such as the Global System for Mobile Communication (GSM), to improve system performance. In general, frequency hopping improves system performance by introducing frequency diversity and interference diversity. The improved performance means that a cellular network can be more heavily loaded if frequency hopping is used than it would have been otherwise. That is, frequency hopping increases the capacity of the network. The impact of frequency hopping on interference between mobiles has become more important due to the trend toward interference limited network planning with tighter frequency reuse.
In a radio telecommunications system that employs frequency hopping, typically a frequency hopping sequence is allocated to a mobile at call setup. Frequency diversity is achieved by transmitting each radio telecommunications signal on the sequence of frequencies over time. Each radio signal is transmitted over a sequence of frequencies because radio signals are often subject to amplitude variations called Rayleigh fading. However, in any given instance Rayleigh fading generally affects radio signals carried on some frequencies more so than other frequencies. Thus, transmitting a radio telecommunications signal over the sequence of different frequencies can increase the likelihood that the signal will be received correctly, as it is unlikely that Rayleigh fading will significantly negatively impact each and every frequency over which the radio telecommunications signal is being transmitted. This benefit exists for signals containing redundancy that enables bit errors experienced during the Rayleigh fading dips to be corrected. Accordingly, signal quality is improved and overall system performance is enhanced.
In addition to fading, a radio signal is often subject to varying degrees of interference caused by traffic (e.g., from closely located mobile stations) on the same frequency (i.e., co-channel interference) and traffic on an adjacent frequency (i.e., adjacent channel interference). If co-channel and/or adjacent channel interference is substantial, the signal quality associated with the radio signal may be severely impacted. In theory, frequency hopping, through the introduction of interference diversity, spreads the co-channel and adjacent channel interference among numerous end-users, such that the co-channel and adjacent channel interference experienced by a particular end-user is diversified. The overall effect is to raise the signal quality across the network, thereby improving overall system performance.
While frequency hopping improves system performance by improving signal quality, frequency reuse is designed to improve system performance by increasing system capacity. More specifically, frequency reuse permits two or more cells to simultaneously use the same frequency, or group of frequencies, so long as the distance (i.e., the “reuse distance”) between the two cells is sufficient to minimize any co-channel interference that might otherwise have an adverse affect on signal quality. However, as the demand for cellular service increases, reuse distances are likely to decrease. And, as reuse distances decrease, co-channel interference is likely to increase.
In order to avoid severe interference between closely located mobile stations (e.g., mobile stations connected to the same base station) using the same set of frequencies, orthogonal frequency hopping sequences are allocated to these mobile stations. Two frequency hopping sequences Shad 1 and S2 are orthogonal if S1(k)≠S2(k) for all time steps k. Orthogonality is depicted by the notation S1⊥S2. The two frequency hopping sequences S1 and S2 are partially orthogonal if the collision probability P(S1(k)=S2(k))=p for some 0<p<1, as depicted by the notation S1⊥p S2. If p=1/N, where N is the number of frequencies used for frequency hopping, full interference diversity is provided. If p=0, there is orthogonality.
In accordance with conventional frequency hopping techniques, frequency hopping sequences can be derived from a reference frequency hopping sequence that is established for the entire system or a part of the system, e.g. a cell. Typically (e.g., as in GSM), the reference frequency hopping sequence is a cyclic or pseudo-random sequence determined by a cell specific parameter (e.g., Hopping Sequence Number “HSN”) and a mobile-specific parameter allocated at call setup or handover. That is, a mobile station, at handover or call set-up, is informed of the cell specific parameter determining the reference frequency hopping sequence and is assigned the mobile-specific parameter (e.g., an available frequency offset) associated with the cell in which the mobile station is operating. The mobile station hops through a sequence of frequencies that are, over time, offset from the reference frequency hopping sequence by a fixed amount that is equal to its assigned frequency offset. In accordance with the GSM standard, each frequency offset is referred to as a Mobile Allocation Index Offset (i.e., MAIO). Depending on the values of these two parameters (HSN and MAIO, for instance) and the parameter that clocks the frequency hopping sequences through time (the TDMA frame number, for example), frequency hopping sequences used by two mobiles may be either identical, orthogonal, or non-orthogonal but with random collisions, so that interference diversity is achieved. Allocating different constant frequency offsets to mobile stations at call setup or handover is an attempt to obtain orthogonality.
Consider, for example, a reference frequency hopping sequence for a group of twelve frequencies denoted 1, 2, 3, . . . , 12 over the time period t1-t10 as being [9, 5, 11, 1, 3, 9, 12, 10, 7, 8]. If a first mobile station operating in a first cell with this reference frequency hopping sequence is, for example, assigned a frequency offset of zero, the first mobile station will hop through the sequence[9, 5, 11, 1, 3, 9, 12, 10, 7, 8] over the time period t1-t10. If a second mobile station operating in a second cell with the same group of twelve frequencies, the same simultaneous TDMA frame number (or other similar timing parameter), and the same reference frequency hopping sequence is assigned a frequency offset of seven, the second mobile station will hop through the sequence [4, 12, 6, 8, 10, 4, 7, 5, 2, 3] over the time period t1-t10. In accordance with conventional frequency hopping techniques, the frequency offset assigned to each mobile station remains constant. Thus, the frequency offset between the first mobile station operating in the first cell and the second mobile station operating in the second cell as described above remains at a fixed value modulo the number of frequencies in the frequency hopping sequence (in this case, it remains fixed at seven modulo twelve).
In order to handle interference limited networks more efficiently, various strategies have been proposed, e.g., synchronization of groups of cells combined with smart frequency hopping sequence allocation to minimize co-channel and adjacent channel interference between mobile stations in selected cells. However, conventional strategies have various limitations. These limitations emanate from the fact that, when dealing with interference, there are two important components to consider. A first component is co-channel interference between mobile stations using the same frequency simultaneously. A second component is adjacent channel interference between mobile stations which are using contiguous frequencies simultaneously.
A first limitation of conventional frequency hopping strategies is lack of adjacent channel interference diversity within a cell and between synchronized cells using the same basic frequency hopping sequence and timing parameter value (i.e., cells which are allocated the same HSNs and simultaneously have the same TDMA frame number in GSM).
A second limitation of conventional frequency hopping strategies is lack of co-channel interference diversity between synchronized cells using the same basic frequency hopping sequence, the same timing parameter value, and the same frequency offsets (e.g., cells using the same HSNs, simultaneous TDMA frame numbers, and MAIOs in GSM).
A third limitation of conventional frequency hopping strategies is insensitivity to discrepancies (e.g., non-randomness) in the basic frequency hopping sequences.
U.S. Pat. No. 6,233,270 to Craig et al, incorporated by reference herein, discloses a mobile station hopping from one frequency to another as a function of the reference frequency hopping sequence plus a frequency offset hopping sequence which it has been assigned. The frequency offset hopping sequence is different in each of the synchronized cells, thereby creating interference diversity. U.S. Pat. No. 6,233,270 thus describes a method to obtain interference diversity between synchronized cells that have been allocated a same frequency hopping sequence.
But, as mentioned before, interference diversity within a cell can also be important, both for synchronized and unsynchronized networks in which either intracell co-channel or intra-cell adjacent channel interference occurs. Intracell interference arises, for example, when a blocked BCCH configuration is employed in a one reuse network and the hardware load is above 0.5 since the use of adjacent frequencies in the same cell cannot be avoided in such a network. In such case it would be beneficial to avoid continuous (ignoring DTX) adjacent channel interference within the cell through frequency hopping.
The challenge, therefore, is to construct frequency offset hopping sequences which, in combination with the original frequency hopping sequences, provide both interference diversity between cells (even if the cells use the same basic frequency hopping sequences, the same sets of frequencies, the same simultaneous timing parameter values, and the same sets of frequency offsets) and adjacent channel diversity (e.g. within a cell), while maintaining orthogonality with respect to co-channel interference.
Assume, for example, a typical GSM frequency hopping method using two basic parameters: the hopping sequence number (HSN) and the frequency offset (MAIO) from the basic frequency hopping sequence. The challenge is to generate frequency offset hopping sequences so that, depending on the input parameters, two frequency offset sequences are either orthogonal with variable frequency offset difference, or non-orthogonal with random collisions so that interference diversity can be provided by the frequency offset sequences alone (irrespective of the basic frequency hopping sequence).
Accordingly, what is needed, and an object of the present invention, is frequency hopping apparatus and technique which provides interference diversity within a cell based on frequency offset.
Advantageously, in one or more of its aspects, the present invention addresses and solves the following issues:                Frequency offset hopping sequence generation that combines the elimination of co-channel interference (orthogonality) and adjacent channel interference diversity between selectable connections using the same set of frequencies.        Improved co-channel interference diversity between non-orthogonal sequences by frequency offset hopping sequences that improve the randomness of non-ideal pseudo-random frequency hopping in existing frequency hopping methods (e.g. GSM).        Frequency offset hopping sequence generation that easily can be combined with any existing frequency hopping generator using e.g. fixed frequency offsets.        Optimal, or near to optimal, adjacent channel interference diversity.        Controllability and flexibility by an appropriate set of parameters that are allocated to network nodes and mobile stations.        Scalability making the method useable and consistent for different network planning and frequency planning configurations.        